gnome doesn't performs neither the associated default action for the filetype of a resource contained in a data: url, nor starts firefox as it does for http: urls.
The following example is displayed as a white square in a black frame when pasted into the location bar of firefox:
data:image/gif;base64,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
but when used on the commandline of gnome-open, it says:
"Error showing url: The default action does not support this protocol."
This prevents neato things like data: urls given to xchat users (once xchat is fixed to recognise data urls) opening in the browser when clicked.
Ah, sorry, a little CSS box overflow there...